How the Eukaryotic Cell Evolved
A. The ancestry of the protists goes back to the evolution of the eukaryotic cell about 1.5 billion years ago.
C. However, the origin of mitochondria, chloroplasts, and perhaps even flagella and cilia, may be explained by the endosymbiotic hypothesis.
1. The endosymbiotic hypothesis explains origin of mitochondria, chloroplasts, and perhaps flagella and cilia.
2. It proposes that aerobic heterotrophic bacteria became mitochondria, and cyanobacteria became chloroplasts (and perhaps spirochetes became flagella and cilia) after being taken up by eukaryotic cells and the prokaryotic and eukaryotic cells developing symbiotic relationships.
3. It has been observed that amoeba infected with bacteria become dependent upon them.
4. Lynn Margulis proposes host cells engulfed bacteria and benefited from ability to utilize O2 or synthesize food.
5. The following evidence supports the endosymbiotic hypothesis:
a. Mitochondria and chloroplasts are similar to bacteria in size and structure.
b. Both organelles are bounded by a double membrane; outer membrane is derived from the engulfing vesicle/vacuole, and inner membrane is derived from the plasma membrane of the original prokaryote.
c. Mitochondria and chloroplasts contain a circular DNA molecule like that of bacteria.
d. Both organelles replicate by splitting.
e. Mitochondria and chloroplasts do have their own ribosomes, which produce some proteins.
f. Ribosomes of both resemble those of bacteria more than they resemble ribosomes of eukaryotic cells.
g. RNA base sequence of their ribosomes suggests a eubacterial origin for chloroplasts and mitochondria.
6. Margulis suggests that the flagella of eukaryotes are derived from a spirochete prokaryote that became attached to a host cell.
a. Eukaryote but not prokaryote flagella have the 9 + 2 pattern of microtubules.
b. The acquisition of basal bodies, which may have become centrioles, may have led to ability to form a spindle during mitosis and meiosis.
30.1. Algae Are Plantlike
A. Classification of Protists
1. Kingdom Protista contains the protists, usually unicellular organisms.
B. Algae
1. Not a taxonomic category, algae refers to aquatic, plantlike organisms carrying out photosynthesis.
2. In oceanic and freshwater ecosystems, algae are part of the plankton and specifically, the phytoplankton.
3. Plankton are freshwater and marine organisms that largely drift along with currents or float near the surface.
4. Phytoplankton are plankton that photosynthesize; they serve as producers for the aquatic community; they also produce much of the oxygen in the atmosphere.
5. Like plants, algae have chloroplasts and their cells are usually strengthened by the presence of a cell wall.
6. Some algae are unicellular, others are colonial, and some are actually multicellular.
7. Multicellular algae are not plants; they lack modifications that protect the gametes and zygote from drying out.
8. Algae are grouped according to their pigments.
C. Green Algae Are Most Plantlike
1. The phylum Chlorophyta contains the green algae and contains about 7,000 species.
2. Most live in the ocean but are more likely found in fresh water and can even be found on moist land.
3. Green algae are believed to be closely related to the first plants because both of these groups have a cell wall that contains cellulose, possess chlorophylls a and b, and store reserve food as starch inside of the chloroplast.
4. Green algae are not always green because some have pigments that give them an orange, red, or rust color.
D. Green Algae That Have Flagella
1. Chlamydomonas is a unicellular green alga less than 25 µm long.
2. It has a cell wall and a single, large, cup-shaped chloroplast containing a pyrenoid for starch synthesis.
3. The chloroplast contains a light-sensitive eyespot (stigma) that helps bring the cell to light for photosynthesis.
4. Two long whiplike flagella project from the anterior end to propel the cell toward light.
5. When growth conditions are unfavorable, Chlamydomonas reproduces sexually.
a. Gametes from two different mating types come into contact and join to form a zygote.
b. A heavy wall forms around the zygote; a resistant zygospore survives until conditions are favorable.
c. Some are heterogametes similar to sperm and egg, a condition called oogamy.
d. In most, the gametes are identical, a condition called isogamy.
E. Green Algae That Are Filamentous
1. Cell division in one plane produces end-to-end chains of cell or filaments.
2. Spirogyra is a filamentous algae that is found on surfaces of ponds and streams.
a. It has ribbonlike chloroplasts arranged in a spiral inside a cell.
b. Two strands may unite in conjugation and exchange genetic material, forming a diploid zygote.
c. The zygotes can withstand the winter; in the spring they undergo meiosis to produce haploid filaments.
3. Oedogonium is another filamentous algae.
a. It has cylindrical cells with netlike chloroplasts.
b. During sexual reproduction, there is a definite egg and sperm.
F. Green Algae That Are Multicellular
1. Multicellular Ulva is often called sea lettuce because of its leafy appearance.
2. The thallus (body) is two cells thick but can be a meter long.
3. Ulva has an alternation of generations life cycle, as do plants.
4. The gametes look alike (isogametes) and the spores are flagellated.
5. Both generations look alike; in true plants, one generation is dominant, sperm and eggs are produced, and spores are not flagellated.
G. Green Algae That Are Colonial
1. A Volvox colony is an example of a colonial algae.
2. A Volvox colony is a hollow sphere with thousands of cells arranged in a single layer.
3. Each Volvox cell resembles a Chlamydomonas cell; a colony arises as if daughter cells fail to separate.
4. Volvox cells cooperate in that flagella beat in a coordinated fashion.
5. Some cells are specialized; they form a new daughter colony that is within the parental colony until it escapes.
6. Sexual reproduction involves oogamy
H. Brown Algae and Golden Brown Algae Are Biochemically Alike
1. Both have chlorophylls a and c in chloroplasts and a carotenoid pigment (fucoxantin) that gives them color.
2. The reserve food is a carbohydrate called laminarin.
I. Brown Algae Are Seaweeds
1. The phylum Phaeophyta includes the brown algae and contains about 1,500 species.
2. They range from small forms with simple filaments to large multicellular (50-100 µm long) seaweeds.
3. Most have an alternation of generations life cycle.
4. Seaweed refers to any large, complex alga.
5. Their cell walls contain a mucilaginous water-retaining material that inhibits desiccation.
6. Laminaria is an intertidal kelp that is unique among protists; this genus shows tissue differentiation.
7. Fucus is an intertidal rockweed; meiotic cell division produces gametes and adult is always diploid.
8. Nereocystis and Macrocystis are giant kelps found in deeper water anchored to the bottom by their holdfasts.
9. Individuals of the genus Sargassum sometimes break off from their holdfasts and form floating masses.
10. Brown algae provide food and habitat for marine organisms, and they are also important to humans.
a. Brown algae are harvested for human food and for fertilizer in several parts of the world.
b. They are a source of algin, a pectinlike substance added to foods to give them a stable, smooth consistency.
J. Golden Brown Algae Include Diatoms
1. The phylum Chrysophyta contains the golden brown algae with about 11,000 species.
2. Some authorities place the diatoms in their own phylum, the Bacillariophyta.
3. Diatoms are the most numerous unicellular algae in the oceans.
4. They are extremely numerous and an important source of food and O2 for heterotrophs in aquatic systems.
5. Diatoms have a cell wall consisting of two silica-impregnated halves or valves.
a. When diatoms reproduce asexually, each received one old valve.
b. The new valve fits inside the old one; therefore, the new is smaller than the original one.
c. This continues until they are about 30 percent of their original size.
d. Then they reproduce sexually; the zygote grows and divides mitotically to form diatoms of normal size.
6. The cell wall has an outer layer of silica (glass) with a variety of markings formed by pores.
7. Diatom remains accumulate on the ocean floor and are mined as diatomaceous earth for use as filters, etc.
K. Dinoflagellates Have Two Flagella
1. The phylum Dinoflagella contains about 1,000 species of dinoflagellates.
2. These algae are bounded by protective cellulose plates.
3. Most have two flagella; one lies in a longitudinal groove and acts as a rudder, the other is located within a transverse groove and the beating causes the cell to spin as it moves forward.
4. Their chloroplasts contain chlorophylls a and c, as do those of golden brown algae.
5. Some species of dinoflagellates are heterotrophic, and it has been suggested that they are really protozoa.
6. They are extremely numerous, up to 30,000 per cubic millimeter, and an important source of ecosystem food.
7. Under certain conditions, those in the genera Gymnodinium and Gonyaulax increase in number enormously and cause a "red tide": they produce a powerful neurotoxin causing paralytic shellfish poisoning.
8. Dinoflagellates also live as symbionts within the bodies of some invertebrates such as corals.
L. Euglenoids Are Flexible
1. The phylum Euglenophyta includes about 1,000 species of euglenoids.
2. Euglenoids are small (10-500 µm) freshwater unicellular organisms.
3. One-third of all genera have chloroplasts; those that lack chloroplasts ingest or absorb their food.
4. Euglenoids grown in absence of light have been known to lose their chloroplasts and become heterotrophic.
5. Their chloroplasts are surrounded by three rather than two membranes; their chloroplasts resemble those of green algae, and are probably derived from a green algae through endosymbiosis.
6. The pyrenoid produces an unusual type of carbohydrate polymer (paramylon) not seen in green algae.
7. They possess two flagella, one of which typically is much longer and than the other and projects out of a vase-shaped invagination; it is called a tinsel flagellum because it has hairs on it.
8. Near the base of the longer flagellum is a red eyespot that shades a photoreceptor for detecting light.
9. They lack cell walls, but instead are bounded by a flexible pellicle composed of protein strips side-by-side.
10. Euglenoids have a contractile vacuole, like certain protozoa, for eliminating excess water.
11. Euglenoids reproduce by longitudinal cell division, and sexual reproduction is not known to occur.
M. Red Algae Are Source of Agar
1. About 4,000 species of red algae are in the phylum Rhodophyta.
2. They are chiefly marine multicellular algae that are generally smaller and more delicate that brown algae.
3. Some are filamentous, but most are branched, having a feathery, flat, or ribbonlike appearance.
4. Coralline algae are red algae whose cell walls have calcium carbonate; they contribute to coral reefs.
5. Sexual reproduction involves oogamy but the sperm are nonflagellated.
6. Their chloroplasts resemble cyanobacteria in that they contain chlorophyll a and the pigment phycobilin.
7. The food reserve is floridean starch and resembles glycogen.
8. Like brown algae, red algae are economically important.
a. Mucilaginous material in cell walls is source of agar used in drug capsules, dental impressions, cosmetics.
b. In the laboratory, agar is a major microbiological media, and when purified, is a gel for electrophoresis.
c. Agar is also used in food preparation to keep baked goods from drying and to set jellies, and desserts.
Protozoa Are Animal-Like
A. Protozoa
1. The term protozoa is a nontaxonomic term used for convenience.
2. Protozoa are typically heterotrophic, motile, unicellular organisms of small size (2-1000 µm).
3. True animals are multicellular, have many kinds of nonreproductive cell, undergo embryonic development.
4. Protozoa usually live in water, but can be found in moist soil or inside other organisms.
5. Protozoa are part of the zooplankton and serve as food for animals in marine and freshwater ecosystems.
6. Some are holozoic (have ability to engulf food whole), others are saprotrophic, and still others are parasitic.
7. Most are unicellular but there are multicellular forms.
8. Food is digested inside food vacuoles.
9. Freshwater protozoa have contractile vacuoles for the elimination of excess water.
10. Asexual reproduction is the rule, but many reproduce sexually during some part of their life cycle.
11. Some form cysts when under adverse conditions; cysts have a protective coat and are metabolically inactive.
12. Once favorable environmental conditions return, cysts are sites for nuclear reorganization and cell division.
13. Protozoa are grouped according to their means of locomotion organelles.
B. Amoeboids and Relatives Move by Pseudopods
1. The amoeboids are in the phylum Sarcodina and include about 40,000 species.
2. They engulf their prey with pseudopods, cytoplasmic extensions formed as cytoplasm streams in one direction.
3. They includes amoebas, foraminifera, and radiolaria.
4. Many amoeboids have shells, as do the foraminifera and radiolaria.
5. Amoeba proteus is a commonly studied member.
6. When amoeboids feed, they phagocytize their food; the pseudopods surround and engulf a prey item.
7. Digestion then occurs within a food vacuole.
8. Freshwater amoeboids, including Amoeba proteus, have contractile vacuoles for eliminating excess water.
9. Entamoeba histolytica is an amoebic parasite that can invade the human intestinal lining.
10. Foraminifera are marine protozoans with an external CaCO3 shell with foramina, holes through which thin pseudopods extend.
a. The pseudopods branch and join to form a net where the prey is digested.
b. Foraminifera are very abundant in marine ecosystems.
c. Over hundreds of millions of years, the CaCO3 shells have contributed to the formation of chalk deposits.
11. The radiolaria have an internal skeleton composed of silica or strontium sulfate.
a. They extend spikelike pseudopods strengthened by microtubules through openings in the shells for feeding.
b. They float near the ocean surface.
C. Ciliates Move by Cilia
1. The phylum Ciliophora contains about 8,000 species of ciliates.
2. Ciliates move by coordinated strokes of hundreds of cilia projecting through tiny holes in a semirigid pellicle.
3. They discharge long, barbed trichocysts for defense and for capturing prey; toxicysts release a poison.
4. Most are holozoic and may ingest food through a gullet and eliminate wastes through an anal pore.
5. During asexual reproduction, ciliates divide by transverse binary fission.
6. Ciliates possess two types of nuclei---a large macronucleus and one or more small micronuclei.
a. The macronucleus controls the normal metabolism of the cell.
b. The micronucleus are involved in sexual reproduction.
1) The macronucleus disintegrates and the micronucleus undergoes meiosis.
2) Two ciliates then exchange a haploid micronucleus.
3) The micronuclei give rise to a new macronucleus containing certain housekeeping genes.
7. Ciliates are diverse.
a. Members of the genus Paramecium are complex.
b. The barrel-shaped didinia expand to consume paramecia much larger than themselves.
c. Suctoria rest on a stalk and paralyze victims, sucking them dry.
d. Stentor resembles a giant blue vase with stripes.
D. Zooflagellates Move by Flagella
1. The phylum Zoomastigophora includes about 8,000 species of zooflagellates.
2. Similar to some unicellular algae, they move by a flagella.
3. These protozoa are covered by a pellicle that is often reinforced by underlying microtubules.
4. Many are symbiotic.
a. Trichonympha collaris lives in the guts of termites mutualistically:
1) T. collaris gains protection and food from the relationship.
2) The zooflagellate contains a bacterium that enzymatically converts wood cellulose to carbohydrates.
b. Giardia lamblia, whose cysts are transmitted through contaminated water, causes severe diarrhea.
c. Trichomonas va is a sexually transmitted organism that infects vagina and urethra of women.
d. Trypanosoma brucei is a trypanosome transmitted by the bite of the tsetse fly; it is the cause of African sleeping sickness.
E. Sporozoa Form Spores
1. The phylum Sporozoa contains about 3,600 species of nonmotile parasitic sporozoa.
2. They contain a complex of organelles that help their invasion of host cells or tissues.
3. Their name recognizes that they form spores at some point in their life cycle.
4. Complicated life cycles alternate between a sexual and an asexual phase, often involving two or more hosts.
5. Pneumocystis carinii causes a type of pneumonia seen primarily in AIDS patients.
a. During sexual reproduction, thick-walled cysts form in the lining of pulmonary air sacs.
b. The cysts contain spores that successively divide until the cyst bursts and the spores are released.
c. Each spore becomes a new organism that can reproduce asexually and can also enter encysted sexual stage.
6. Plasmodium vivax causes one type of malaria; it is the most widespread human parasite.
a. If a human is bitten by a female Anopheles mosquito, the parasite eventually invades the red blood cells.
b. Chills and fever appear when red blood cells burst and release toxin into blood.
7. Toxoplasma gondii causes toxoplasmosis, particularly in cats but also in humans.
a. In pregnant woman, the parasite can infect the fetus and cause birth defects.
b. In AIDS patients, it can infect the brain and cause neurological symptoms.
Slime Molds and Water Molds Are Funguslike
A. Similarities to Fungi
1. Like fungi, they have a filamentous body and are saprotrophic.
2. Slime molds produce nonmotile spores, have an amoeboid stage and are heterotrophic by ingestion.
3. Water molds are unlike fungi in that they produce diploid (2n) zoospores.
B. Slime Molds Are Amoeboid
1. The phylum Gymnomycota includes 560 species of slime molds.
2. They look like molds but their vegetative state is mobile and amoeboid.
3. They are heterotrophic by ingestion of organic material and bacteria.
4. If conditions are unfavorable, they produce and release nonmotile spores resistant to environmental extremes.
5. When conditions become more favorable, the spores germinate, releasing cells that begin the life cycle again.
C. Plasmodial Slime Molds Are A Multinucleated Mass
1. Plasmodial slime molds exist as a plasmodium.
a. This diploid multinucleated cytoplasmic mass creeps along, phagocytizing decaying plant material.
b. Fan-shaped plasmodium contains tubules formed by concentrated cytoplasm in which more liquefied cytoplasm streams.
c. Under unfavorable environmental conditions (e.g., drought), the plasmodium develops many sporangia, within which spores are produced by meiosis.
d. When mature, the spores are released and can survive until more favorable environmental conditions return, at which time they germinate, each releasing a haploid flagellated cell or an amoeboid cell.
e. Two flagellated or amoeboid cells fuse to form diploid zygote; this produces a multinucleated plasmodium.
D. Cellular Slime Molds Are Single Cells
1. Cellular slime molds usually exist as individual amoeboid cells that live by phagocytizing bacteria and yeast.
2. As food runs out, amoeboid cells release a chemical that causes them to aggregate into a pseudoplasmodium.
3. The pseudoplasmodium stage is temporary and eventually gives rise to a sporangium that produces spores.
4. When mature, the spores are released.
5. The spores can survive until more favorable environmental conditions return, at which time they germinate.
6. Spore germination involves release of haploid amoeboid cells, which is again the beginning of asexual cycle.
7. Asexual cycle is known to occur under very moist conditions.
E. Water Molds Are Filamentous
1. The phylum Oomycota includes the water molds with about 580 species.
2. Aquatic water molds parasitize fishes, forming furry growths on their gills.
3. Some terrestrial water molds parasitize insects and plants; water mold was responsible for Irish potato famine.
4. Water molds have a filamentous body but cell walls are composed largely of cellulose.
5. During asexual reproduction, they produce diploid motile spores (2n zoospores).
6. Unlike fungi, the adult is diploid; gametes are produced by meiosis.
7. Eggs are produced in enlarged oogonia.